As is known, one of the services provided by a coffee machine is the dispensing of superheated vapour for heating drinks and in particular for heating the milk and optionally frothing it with an extremely fine foam in order to prepare a cappuccino.
The conventional method of bringing about the heating and frothing of the milk involves introducing into the jug, which contains the milk to be heated, a tube which, in the specific technical field, is referred to as a nozzle and which is connected to a valve which shuts off the vapour from a pipe which is connected to a vapour generator.
The vapour released by the valve is discharged by the end-piece of the nozzle and, upon contact with the cold milk in the jug, condenses and gives off sensible heat and latent heat. At the same time, the operator with suitable movements of the jug, using the kinetic energy of the vapour which, urged by the internal pressure of the generator, is introduced into the milk at a given rate, also brings about the frothing by incorporating air and mixing the mixture of air and milk until an extremely fine foam is formed.
The end-piece of the nozzle is important in optimising the process of heating and frothing. In fact, in order to form a milk foam with the characteristics of texture and persistence necessary for producing a cappuccino, it is necessary to mix the mixture of milk and air for a time of at least 15 seconds whilst the milk is in the temperature range between 30° C. and 60° C.
In order to operate so as to comply with the conditions set out above, the end-piece of the nozzle is provided with vapour discharge holes having a specific cross-section and orientation.
Milk drinking customs vary greatly from country to country. For a single drink, for example, in the United States, there are also used quantities greater than 250 cm3 whilst, in Italy, for a cappuccino there are used quantities of approximately from 80 cm3 to 100 cm3. Consequently, the quantities of milk to be heated for each individual operation may vary, for example, from 100 cm3 in order to prepare a single cappuccino up to a liter in order to prepare four drinks of the latte americano type. In order to provide efficient service, it is necessary to adapt the flow of vapour to the quantity of milk to be heated so as to froth the milk before exceeding the critical temperature of 60° C. without impairing the rapidity of service when heating large quantities and, in the case of small quantities, in order not to take the temperature beyond the critical threshold for the frothing without having had time to mix the mixture of milk and air.
In the case of operations carried out manually, the operator carries this adjustment out by acting on the flow of vapour of the valve by means of an adjustment and control knob; however, that operation involving the flow and pressure inside the nozzle being varied also causes a variation in the velocity of the vapour being discharged from the holes of the end-piece with a resultant loss in the efficiency of the action of the air being incorporated and mixed.
Therefore, the quality of frothing obtained tends to move increasingly further away from the ideal as the quantities heated become increasingly small with respect to the quantities for which the end-piece and the holes thereof were sized. However, when the quantities heated are higher with respect to those for which the holes of the end-piece have been sized, the times increase in an unacceptable manner.
For example, in the case of a machine with a nozzle which is provided with an end-piece having four holes, each having a diameter of 2 mm, and which is sized to heat and froth 1 liter of milk in approximately 1 minute, if it were instead desirable to heat and froth with this nozzle a quantity of only 100 cm3, in order to mix milk and air for a time of no less than 15 seconds without simultaneously increasing the temperature of the milk above 60° C., it would be necessary to reduce the quantity of vapour, by controlling the relevant valve, to such a flow that the velocity of the vapour through the four holes of 2 mm would thereby become so low as to compromise the efficiency of the effect of conveying and mixing the air obtaining frothing of poor quality as a result.
For the quantity of milk indicated, however, the arrangement considered optimum would be a configuration of the end-piece provided with three or four holes having a diameter between 1.2 and 1.5 mm.
In order to solve the above-mentioned problems, it is known in the specific technological field to provide the coffee machines with end-pieces for vapour nozzles in the form of an accessory to be fitted to the dispensing end of the nozzle so as to be able to have, with the end-piece being replaced, different numbers of holes with different diameters.
With the development of automation, in place of the manual valves for adjusting the flow of vapour to the nozzle, there have been developed devices such as the ones described, for example, in European Patents EP 1501398 B1 and EP 1706905 B1, which bring about the operations of heating and frothing in a completely automated manner.
In those automatic devices, in addition to the functional requirements of the manual devices, the end-pieces of the nozzles further have to be sized so as to also receive a thermocouple in order to measure the temperature reached by the milk during heating and frothing.
Construction of those end-pieces involves the use of complex equipment and troublesome processing cycles so that those end-pieces in the form of replaceable accessories constitute a costly component of the nozzle and consequently of the coffee machine.